I recently picked up a cheap back issue of Action Comics #842, dated October 2006. In it, a giant alien, the "Auctioneer," is menacing the Earth. He's mostly oblivious to the frantic human activity around him, but Superman manages to attract his attention. The alien is delighted to have found a Kryptonian, and instructs his equipment to scan the Earth for others. Superman overhears him comment that, in total, three were found, and is surprised, since the only other Kryptonian he knows about is Supergirl. However, the identity of the third Kryptonian is not revealed in that issue.
So, who was it?